Chemical Equations and Chemical Reactions. Chapter 8.1

Transcription

1 Chemical Equations and Chemical Reactions Chapter 8.1

2 Objectives List observations that suggest that a chemical reaction has taken place List the requirements for a correctly written chemical equation. Write a word equation and a formula equation for a given chemical reaction Balance a formula equation by inspection

3 Chemical Reaction - process by which one or more substances are changed into one or more different substances Reactants - the original substances Products - the resulting substances Law of Conservation of Mass - the total mass of reactants must equal the total mass of products for any given chemical reaction

4 Chemical Equation - represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction

5 Indications of a Chemical Reaction 1. Release of energy as heat and/or light 2. Production of a gas 3. Formation of a precipitate 4. Color change

6 Characteristics of Chemical Equations 1. Must represent known facts 2. Must contain the correct formulas for reactants and products 3. Law of Conservation of Mass must be satisfied

7 Word Equations An equation where the parts of a reaction are represented with text Hydrogen mixed with oxygen will produce water

13 What a Chemical Equation Does NOT tell us An equation gives no indication of whether a reaction will actually occur Chemical equations give no information about the speed at which reactions occur Equations do not give any information about how the bonding between atoms or ions changes during the reaction

17 Balancing Chemical Equations 3. Balance the formula equation according to the law of conservation of mass. Balance the different types of atoms one at a time. First balance the atoms of elements that are combined and that appear only once on each side of the equation. Balance polyatomic ions that appear on both sides of the equation as single units. Balance H atoms and O atoms after atoms of all other elements have been balanced.

18 Balancing Chemical Equations 4. Count atoms to be sure that the equation is balanced. 2H 2 O(l) 2H 2 (g) + O 2 (g) (4H + 2O) = (4H) + (2O) If the coefficients do not represent the smallest possible whole-number ratio of reactants and products, divide the coefficients by their greatest common factor in order to obtain the smallest possible whole-number coefficients.

19 Helpful Balancing Hints 1. Never touch subscripts when balancing equations.that will change the composition and therefore the substance itself. 2. Check to be sure that you have included all sources of a particular element that you are balancing on a particular side. There may be two or more compounds that contain the same element on a given side of an equation.

20 Helpful Balancing Hints 3. Adjust the coefficient of lone elements near the end of the balancing act since any change in their coefficient will not affect the balance of other elements 4. Diatomic elements (H,N,O, & Group 17) always appear as 2 atoms i.e O 2

21 Assignment Na 2 O + H 2 O NaOH Aluminum sulfate, an ingredient in antiperspirants, is made by the reaction of solid aluminum oxide with aqueous sulfuric acid. In addition to aqueous aluminum sulfate, water is also produced. Write a balanced chemical equation for the reaction. NH 3 (g) + O 2 (g) NO(g) + H 2 O(g) Pb 3 O 4 (s) + HNO 3 (aq) Pb(NO 3 ) 2 (aq) + PbO 2 (s) + H 2 O(l) One of the most important uses of sulfuric acid is in the production of phosphoric acid for use in the making of fertilizers. Solid calcium phosphate is reacted with aqueous sulfuric acid to form phosphoric acid and solid calcium sulfate. Write a balanced chemical equation for the reaction. When calcium carbonate, CaCO 3, is heated strongly, it decomposes to form calcium oxide, CaO, and carbon dioxide, CO 2. Write a balanced chemical equation for this reaction.

22 Types of Chemical Reactions Chapter 8.2

23 Types of Chemical Reactions Objectives: Define and give general equations for synthesis, decomposition, single-displacement, and doubledisplacement reactions. Classify a reaction as a synthesis, decomposition, single-displacement, double-displacement, or combustion reaction. Predict the products of simple reactions given the reactants.

24 Types of Chemical Reactions The classification scheme described in this section provides an introduction to five basic types of reactions: synthesis decomposition single-displacement double-displacement combustion reactions

25 Synthesis Reactions Two or more substances combine to form a new substance E + E C A + B AB 8 Fe + S 8 8 FeS Not a practical method for creating compounds

27 Synthesis Reactions Many oxides of nonmetals in the upper right portion of the periodic table react with water to produce oxyacids. example: SO 2 (g) + H 2 O(l) H 2 SO 3 (aq) Certain metal oxides and nonmetal oxides react with each other in synthesis reactions to form salts. example: CaO(s) + SO 2 (g) CaSO 3 (s)

30 Decomposition Reactions Electrolysis - The decomposition of a substance by an electric current example: 2H 2 O(l) electricity 2H 2 (g) + O 2 (g) Oxides of the less-active metals, which are located in the lower center of the periodic table, decompose into their elements when heated. example: 2HgO(s) 2Hg(l) + O 2 (g)

32 Single Displacement Reactions A metal will not always replace another metal in an aqueous compound Metal must be more reactive than the metal it replaces NR indicates no reaction takes place Halogens only replace elements below them on the periodic table

33 Activity Series of the Elements Chapter 8.3

34 Activity Series Activity - the ability of an element to react The more readily an element reacts with other substances, the greater its activity is Activity Series - a list of elements organized according to the ease with which the elements undergo certain chemical reactions For metals, greater activity means a greater ease of loss of electrons, to form positive ions. For nonmetals, greater activity means a greater ease of gain of electrons, to form negative ions.

35 Chapter 8 Section 3 Activity Series of the Elements The order in which the elements are listed is usually determined by single-displacement reactions. The most-active element is placed at the top in the series. It can replace each of the elements below it from a compound in a single-displacement reaction. Activity series are used to help predict whether certain chemical reactions will occur. Activity series are based on experiment.

36 Activity Series

37 Single Displacement Reactions Displacement of a Metal in a Compound by Another Metal To replace a metal, the replacing metal must be more active than the metal it is replacing Aluminum is more active than lead 2Al(s) + 3Pb(NO 3 ) 2 (aq) 3Pb(s) + 2Al(NO 3 ) 3 (aq)

38 Single Displacement Reactions Displacement of Hydrogen in Water by a Metal The most-active metals, such as those in Group 1, react vigorously with water to produce metal hydroxides and hydrogen. 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) Less-active metals, such as iron, react with steam to form a metal oxide and hydrogen gas. 3Fe(s) + 4H 2 O(g) Fe 3 O 4 (s) + 4H 2 (g)

39 Single Displacement Reactions Displacement of Hydrogen in an Acid by a Metal The more-active metals react with certain acidic solutions, such as hydrochloric acid and dilute sulfuric acid, replacing the hydrogen in the acid The reaction products are a metal compound (a salt) and hydrogen gas Mg(s) + 2HCl(aq) H 2 (g) + MgCl 2 (aq)

42 Double-Displacement Reactions Formation of a Precipitate The formation of a precipitate occurs when the cations of one reactant combine with the anions of another reactant to form an insoluble or slightly soluble compound example: 2KI(aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2KNO 3 (aq) The precipitate forms as a result of the very strong attractive forces between the Pb 2+ cations and the I anions.

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